Fix 8 bugs from code review

B#1 — sigCh shared across goroutines, SIGINT unreliable
  → context.WithCancel: main loop cancels ctx on SIGINT,
    4 WS goroutines select on ctx.Done() instead of shared sigCh

B#3 — restoreOpenPositions missing LastScaleAt
  → Set LastScaleAt = tr.OpenedAt on restore so scale-in cooldown works

B#4 — dYdX heartbeat goroutine leaks on reconnect
  → Added stopHeartbeat chan + heartbeatMu mutex; close old channel
    before spawning new heartbeat goroutine

B#5 — GetBitgetSize fmt.Sprintf rounds up, may exceed amountUSD
  → Added math.Floor(sz*multiplier)/multiplier before format to round
    DOWN to nearest valid step size for every coin

B#6 — netProfit and CalcNetProfit duplicate formula
  → scanner.go netProfit now delegates to exchange.CalcNetProfit

B#7 — Aevo Run callback only 2 params, incompatible with startExchange
  → Changed to 4-arg callback func(coin, price, bid, ask) with bid=ask=0

B#8 — parseFloat uses fmt.Sscanf (slow, locale-sensitive)
  → Replaced with strconv.ParseFloat

B#9 — dYdX receives hlSymbols instead of its own symbol list
  → Added dydxSymbols var, built from c.HL like other exchanges

exchange/connector.go

@@ -1,8 +1,8 @@
 package exchange
 
 import (
-	"fmt"
 	"log"
+	"strconv"
 	"time"
 
 	"github.com/gorilla/websocket"
@@ -148,9 +148,10 @@ func (pc *PriceConnector) SendJSON(v interface{}) error {
 }
 
 // Helper: parse float from string
+// B#8: use strconv.ParseFloat instead of fmt.Sscanf
 func parseFloat(s string) float64 {
-	var f float64
-	if _, err := fmt.Sscanf(s, "%f", &f); err != nil {
+	f, err := strconv.ParseFloat(s, 64)
+	if err != nil {
 		return 0
 	}
 	return f